Advanced solution polymerization processes, as described in U.S. patent application Ser. No. 11/961,583 involve increased monomer concentrations in the polymerization reactor(s) delivering polymeric products with improved properties, such as higher molecular weight and melting/crystallization temperatures. Similar advantages are disclosed for supercritical polymerization as described in U.S. patent application Ser. No. 10/667,585. For the case of the production of the isotactic polypropylene using the advanced solution and supercritical polymerization process, the higher propylene concentration enables the production of highly crystalline isotactic polypropylenes with sufficiently low melt flow rates. Higher monomer concentrations also enable higher polymer concentrations in the reactor effluent thus reducing the monomer recycle ratio and with it the cost of monomer and solvent recycle. Both processes are performed in homogeneous dense fluid polymerization systems.
Homogeneous polymerization processes, such as supercritical, solution, and advanced solution polymerization, advantageously employ phase separators for recovering the bulk of the low molecular weight monomer and solvent components from the reactor effluent for recycle as described in U.S. patent application Ser. No. 11/854,93. The separation process involves heating the reactor effluent before letting its pressure down to rapidly cross the fluid-fluid phase separation boundary from the single-phase to the two-phase region in the phase diagram. The heating step needs to introduce enough heat to cover the enthalpy of evaporation of the solvent and monomer remnants after phase separation, plus the heat required to stay above the melting point of the polymer, as described in U.S. Pat. No. 7,163,989. With solution polymerization processes, for example, this entails raising the temperature of the reactor effluent from around 50 to 90° C. to about 160 to 200° C. in a heat exchanger. The maximum temperature typically is limited by the reactor effluent crossing into the two-phase region upon heating, due to the reduced solvency at increasing temperatures. If a polymer-rich phase forms in the heat-exchanger, it tends to foul the heat exchanger, resulting in a substantial deterioration of the heat transfer coefficient and other plant operation problems.
This limitation becomes even more severe in the advanced solution and supercritical polymerization processes described in U.S. patent application Ser. Nos. 11/961,583 and 10/667,585 respectively, because the solvency of the reactor effluent is reduced by the higher monomer concentration, i.e., the monomers are poorer solvents for the product polymer than the inert solvent (typically paraffinic hydrocarbons, such as hexanes or octanes, etc.). This issue may limit the temperature to which the reactor effluent can be heated and/or the maximum polymer concentration in the reactor effluent, and correspondingly, may lead to difficulties in removing the solvent and monomer remnants from the final product and increase the volume and cost of recycle, and reduce plant capacity for a given separation and devolatilization train.
Hence, there is a need for an improved monomer separation and recycle process that can reduce or even eliminate these limitations caused by the increased monomer concentrations in the reactor effluent. The improvements can be particularly advantageous for the supercritical and advanced solution polymerization processes due to their typically lower concentration of solvents in the polymer-containing reactor effluents resulting in lower fluid-fluid phase separation temperatures as compared to the conventional solution processes practiced today. In regard to supercritical and advanced solution polymerization processes, an improved monomer separation and recycle process would allow all the benefits of the increased monomer concentration, namely the improved product capability, and reduced cost of monomer and solvent recycle to be captured. Nonetheless, the disclosed monomer/solvent separation-recycle process configurations provide advantages even in a conventional solution polymerization plant operating with higher solvent concentrations by providing a reduced risk of fouling in the separation-recycle train.